ring, planetary

An annulus of material, in the form of countless
numbers of small objects, in orbit around a planet. Ring matter –
whether it is dust particles or huge boulders – will orbit a planet
without coalescing into a single body if it remains within a certain distance,
known as the Roche limit. Any bodies
that orbit outside this limit won't be adversely affected by the gravity
of the parent planet and may therefore accrete into a larger body. All four
giant planets in the Solar System have ring systems, though only Saturn's
is bright enough to be seen at visible wavelengths from Earth. The various
rings have different proportions of ice and rock, and different distribution
of particle sizes.